Rhodium-catalyzed hydrogenation phosphine ligands

Recently, two new P- and C-chiral monodentate phosphines 13 were reported. The ligands were applied in a number of transition metal-catalyzed reactions, though ee-values in the rhodium-catalyzed hydrogenation of N-acyl dehydrophenylalanine were only moderate [37]. 

Whitesides and co-workers [40] synthesized the nonionic surfactant phosphines 9 (n = 2 x = 0 m = 12, 16 R = Me), 12, 13, 14 and used them as ligands in rhodium-catalyzed hydrogenation of water-immiscible starting materials such as cyclohexene in a two-phase system. 

In the aforementioned work, ligands are attached covalently to a polymer. An interesting non-covalent attachment was reported early on by Wilson and Whitesides [15]. An achiral phosphine ligand linked to a biotin unit (Structure 6) interacted specifically with the protein avidin in aqueous solution to impose stereoselectivity in catalytic hydrogenation. In the rhodium-catalyzed hydrogenation of a-acetamidoacrylic acid in buffered aqueous solution, ee values of up to 40% were observed at TONs of 500 in a 48 h mn. Catalyst recovery was not a motivation for this work, but during workup the catalyst was separated from the low molecular weight products by ultrafiltration. 

Scheme 2 Rhodium catalyzed hydrogenation of aldehydes using a pyrrolo-acylguanidinium containing a phosphine ligand.

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

It thus came as a surprise that in the year 2000, three groups independently reported the use of three new classes of monodentate ligands (Scheme 28.2) [12], The ligands induced remarkably high enantioselectivities, comparable to those obtained using the best bidentate phosphines, in the rhodium-catalyzed enantioselective alkene hydrogenation. All three being based on a BINOL backbone, and devoid of chirality on phosphorus, these monophosphonites [13], monophosphites [14] and monophosphoramidites [15] are very easy to prepare and are equipped with a variable alkyl, alkoxy, or amine functionality, respectively. 

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

Th effect of pH on the rate of hydrogenation of water-soluble unsaturated carboxylic acids and alcohols catalyzed by rhodium complexes with PNS [24], PTA [29], or MePTA r [32] phosphine ligands can be similarly explained by the formation of monohydride complexes, [RhHPJ, facilitated with increasing basicity ofthe solvent. 

The discovery of Wilkinson complex, RhCl[P(C6H5)3]3, acting as an effective catalyst for hydrogenation of olefins opened the door for developing asymmetric reaction catalyzed by rhodium complexes with a chiral phosphine ligand. 

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